1. Field of the Invention
The present disclosure relates to electron detectors and electron beam apparatus.
2. Description of the Background Art
Scanning electron microscope (SEM) instruments may utilize bright-field and dark-field detectors to enhance the contrast of the defects of interest in the image.
A bright-field detector detects those secondary electrons that are emitted from the sample at relatively low polar angles relative to the normal to the surface. A typical upper limit for the polar angle relative to the surface normal for bright-field electrons may be considered to be forty-five (45) degrees, for example. An image formed using the signal from a bright-field detector (i.e. a bright-field image) is similar in appearance to that of a standard top-down image, such as that obtained by a critical dimension SEM (CD-SEM), for example.
A dark-field detector is to detect those secondary electrons that are emitted from the sample at relatively high polar angles relative to the normal to the surface. A typical lower limit for the polar angle relative to the surface normal for dark-field electrons may be considered to be forty-five degrees, for example. An image formed using the signal from a dark-field detector (i.e. a dark-field image) gives the appearance of being illuminated from a side angle and shows enhanced edge contrast. Small particles, scratches, dents, or bumps that are not at all or hardly visible in the regular or bright-field image, tend to show clearly in the dark-field image. Multiple dark-field detectors may be used to allow one to view the sample at different apparent directions of illumination so as to more clearly highlight the features of interest.
It is highly desirable to improve SEM instrumentation. More particularly, it is highly desirable to improve bright-field and dark-field capabilities of SEM instrumentation.